1. Field of the Invention
The present invention relates to an electronic device that uses a lead-free solder (solder that contains at most a trace amount of lead) and, more particularly to an electronic device fabricated by solder bonding using a temperature hierarchy that is effective in mounting a module formed of the electronic device or the like.
2. Description of Related Art
In bonding using Sn—Pb-base solders, temperature-hierarchical bonding has been used. In this bonding technique, parts are soldered first at a temperature between 330 degrees centigrade and 350 degrees centigrade using solder for high-temperature soldering such as Pb-rich Pb-5 mass % Sn solder (melting point: 314-310 degrees centigrade) or Pb-10Sn mass % solder (melting point: 302-275 degrees centigrade). Thereafter, another bonding is performed without melting the soldered portion using solder for low-temperature soldering such as Sn-37Pb eutectics (183 degrees centigrade). (Hereafter, the indication of “mass %” is omitted and only the numeral is recited). This temperature-hierarchical bonding is used in the fabrication of semiconductor devices in which chips are die-bonded as well as in the fabrication of semiconductor devices that use flip chip bonding, etc. For example, temperature-hierarchical bonding is necessary for forming BGA, CSP, WL-CSP (Wafer Level CSP), a multi-chip module (abbreviated as MCM), and the like. In the semiconductor fabrication process, it has become important to provide temperature-hierarchical bonding that can perform soldering for bonding parts inside a semiconductor device and another soldering for bonding the semiconductor device, itself, to a substrate.
On the other hand, with respect to some products, there have been cases in which bonding at a temperature of not more than 290 degree centigrade is requested in consideration of the heat-resistance limit of parts. As solders having the compositions that fall in a composition range for high-temperature soldering suited to this requirement in conventional Sn—Pb-base solders, a Pb-15Sn solder (liquidus temperature: 285 degrees centigrade) and solders having similar compositions are considered. However, when the Sn content is above this level, low-temperature eutectics (183 degrees centigrade) precipitate. Furthermore, when the Sn content is below this level, the liquidus temperature rises; consequently, bonding at a temperature of ≦290 degrees centigrade becomes difficult. For this reason, even when a secondary reflow solder for bonding to a printed circuit board is a eutectic Sn—Pb-base solder, the problem of remelting of high-temperature solder bonds is unavoidable. When Pb-free solders are used for secondary reflow, bonding is performed at a temperature that falls in a range of 240-250 degrees centigrade. This temperature is about 20-30 degrees centigrade higher than necessary for bonding using eutectic Sn—Pb-base solders. Accordingly, bonding at a temperature of ≦290 degrees centigrade using Pb-free solder has further difficulties.
More specifically, at present, there is no high-temperature Pb-free soldering material that permits temperature-hierarchical bonding at a soldering temperature ranging from 330 to 350 degrees centigrade or at a temperature level of 290 degrees centigrade.
This situation is described in detail below. At present, Pb-free solders are being used increasingly in many applications to address environmental issues. With respect to Pb-free solders for soldering parts to printed circuit boards, eutectic Sn—Ag-base solders, eutectic Sn—Ag—Cu-base solders and eutectic Sn—Cu-base solders are becoming the mainstream. As a result, the soldering temperature in surface mounting is usually in a range of 240 to 250 degrees centigrade. However, there is no Pb-free solder for a temperature hierarchy on the higher-temperature side that can be used in combination with these eutectic Pb-free solders for surface mounting. The solder composition that is the most probable candidate for higher-temperature-side solder, is a Sn-5Sb solder (240-232 degree centigrade). However, taking into account the irregularities of temperature and the like on a substrate in a reflow furnace, no highly reliable lower-temperature-side solder exists that can bond without melting the Sn-5Sb solder. On the other hand, although an Au-20Sn solder (melting point: 280 degrees centigrade) is a known high-temperature solder, its use is limited because it is a hard material and its cost is high. Especially in bonding a Si chip to a material having an expansion coefficient that largely differs from an expansion coefficient of the Si chip, or in bonding a large-sized Si chip, Au-20Sn solder is not used because it is hard and may break the Si chip.